Karst monadnock fine characterization and reservoir control analysis: A case from Ordovician weathering paleokarst reservoirs in Lungu area, Tarim Basin, NW China

Based on the fine characterization of weathering crust paleokarst landform of the Ordovician Yingshan Formation in Lungu area, Tarim Basin, the size, number and reservoir controlling characteristics of karst monadnocks have been investigated quantitatively. Impression method and karst monadnock volume formulas were employed to characterize the various levels of karst microtopography, to work out the number, relative relief, area, volume and other parameters of monadnocks, and analyze the reservoir development situation at different parts of monadnock. The results showed that the weathering crust karst reservoirs in Lungu area are 140-160 m in relative altitude, and thinning from karst highland to karst slope and terrace. Two hundred fifty-two karst monadnocks have been identified in the study area. Among them, type I and type II monadnocks are fewer in number, but larger in bulk volume, and are favorable reservoir development zone. The karst monadnocks have a high coincidence ratio with current structural high points of 96.42%, and the monadnock morphology has remained basically unchanged during the late tectonic movement, thus “karstification-reservoir-accumulation” configuration has been kept good. In the main part (core) of karst monadnocks, there are cave, vug, fracture-vug, and fracture reservoirs, thicker high quality reservoir, and all reservoir parameters are better than those of karst monadnock wings. It is found that the relative amplitude of karst monadnock is positively correlated with single well productivity, and the scale and position of monadnock are closely related with productivity.